The refractive properties of an eye depend, inter alia, from the pupil diameter. This means that a refraction measurement that was carried out under normal light conditions is longer valid for poor light conditions because the size of the pupil has increased. If, therefore, a person owns a spectacle that was optimized for daylight, the visual acuity at twilight, and, in particular during night driving in a vehicle, may be reduced because the spectacle is not optimized for these light conditions. As a consequence, it may happen that persons see very well at daytime, be it with or without a spectacle, whereas they are almost blind at night because the daytime refraction of their eyes is entirely different from their nighttime refraction.
In the prior art several methods and apparatuses have already been suggested for testing the visual behavior during twilight and in the darkness. Other phenomena are related thereto, like adaptation, i.e. the ability of the eye to adapt itself more or less quickly from bright ambient conditions to dark conditions.
For measuring the visual impression, in particular the visual acuity of an eye, subjective as well as objective approaches are known. The visual impression comprises the entire perception of the test person, i.e. including potential peripheral limitations in the field of his/her vision (tunnel vision) and limited areas of vision loss within the field of vision (skotoma). The visual acuity is related solely to the sharp vision, i.e. the recognition of details, independent from the above limitations.
With the subjective approach certain visual marks are presented to the test person for him/her to look at through a test spectacle into which test spectacle lenses are inserted one after the other by an examination person. The test person then informs the examination person with which test spectacle lens he/she can best see the visual mark, in particular in the sharpest way. Accordingly, the subjective ruling of the test person influences the examination.
With the objective approach, in contrast, one, for example, measures the refraction of the eye without any interaction with the test person, for example with a wavefront analyzer. This method, accordingly, results in an objectively optimum result which, however, in practice is not always congruent with the subjective impression of the test person. Therefore, both methods are often combined in order to achieve an optimum result.
In the field of apparatuses for measuring the vision of an eye, one distinguishes between those, in which the test person sits freely in a room and looks at visual marks appearing at a distance of several meters, further those configured as box-type tabletop devices into which the test person looks through two eyepieces, and finally those which the test person carries as goggles or helmets. The two last-mentioned apparatuses have the disadvantage that they may result in a so-called device-myopia because the test person does not look under normal conditions.
From German patent specification 30 03 588 C2 it is known to test twilight vision, flare stability and adaptation ability by means of an apparatus in which certain visual marks of predetermined brightness are projected onto a screen at predetermined ambient brightness. The test person is located at a distance of at least three meters from the screen. The visual marks are of known character, and are, for example, configured as Landolt rings. With such an apparatus a precise examination and determination of night visual acuity is impossible.
From U.S. Pat. No. 3,328,113 a device for testing night vision is known. With this prior art device peripheral light points are presented to the test person as visual marks. The light points are generated by means of a flickering light source and a rotatable aperture disk arranged in front thereof in connection with radial slits in the front plate of the device. The aperture disk is provided with openings of different diameter in the range of between 0.6 and 14 mm having different distances from the aperture disk axis of rotation. In this way one can present light points of different size and different distance from the axis of rotation by rotating the aperture plate arranged immediately behind the radial slits. A chin rest for the test person is arranged at about 40 cm in front of the device. With this prior art device one can, therefore, also just test the vision at darkness. A visual acuity measurement is not provided.
Besides the above, the prior art also describes tabletop devices (German disclosure document DE 23 21 570; U.S. Pat. No. 5,870,168; Japanese disclosure document JP 2001-087225 A) as well as helmet- or goggle-type apparatuses (German disclosure document DE 43 26 760 A1; German patent specification DE 197 29 102 C2) used, inter alia, for testing twilight vision, however use conventional visual marks, and, some of them flaring means for testing adaptation capability.